NOT Bondo, and NOT wood filler. They are both too soft. We fill the slots with a contrasting wood veneer, usually maple. We get thick veneers, widen the fret slots to fit the veneers, and inlay them with cyanoacralate (super glue, but the stuff we use is very high quality).
You also never want to finish a fretless fingerboard, as the finish will wear away much faster than bare rosewood or ebony. Even epoxy. It is all about the wood.
Of course, Jaco never filled the slots with anything.
Light
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